<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        try  {
            alert("Result:" + eval(prompt("Enter an expression:","")));
        }

        catch(exception) {
            alert(exception);
        }
    </script>

</head>
<body>

</body>
</html>